The present invention relates to pieces made of refractory material, permeable to gases.
It is known that certain industrial practices require a gas to go through a refractory piece.
Such is the case, for example, in the field of metal technology, particularly steel, where, for metallurgical purposes, it has already been proposed that gases be blown into a bath of molten metal in a metallurgical container through permeable refractory pieces, generally hexahedral in shape, similar to that of the bricks usually used for the refractory lining of the inside of the container. These pieces are incorporated in the refractory lining at a level below that of the surface of the bath, and more generally on the bottom, so that one of its surfaces is in contact with the molten metal and the opposite surface is available for introducing a gas under pressure.
In this type of application, where the molten metal is in contact with the blast surface of the permeable piece, it is of course desirable that the permeability of the latter be "selective", that is to say, that it permit the gas to go through in one direction without, as a result, causing infiltrations of liquid metal in the opposite direction, if possible even in the absence of a blast of gas. For this purpose, the manufacture of permeable refractory pieces is known with the use of a raw material of a special particle size, molded and sintered, so that the mass obtained has a statistically isotropic open microporosity (French Pat. Nos. 1.094.809 and 1.162.737).
Furthermore, it is equally desirable that this permeability be also "oriented", because the flow of blown gas has to be directed in such a way that it enters under pressure in the porous piece through one side and goes out through the opposite aide in contact with the molten metal, while the other surfaces must remain completely tight to prevent too much side loss of the gas which naturally tends to grow with the height of the piece. For this purpose, it has been already proposed that this piece be suitably enclosed in a tight receptacle, consisting for example of a metal casing (French Pat. No. 1.031.504), or a layer of refractory concrete rendered tight by selecting a particle size finer than that of the central area (French Pat. Nos. 1.183.569 and 1.350.751).
Elements of this type with a composite structure are relatively easy to make. However, their use poses certain problems, particularly in relation to the phenomenon of differential expansion under heat between the porous core and the tight surrounding periphery, leading to the undesirable formation between the two of preferential passages for the gas which is blown in, with all of the resulting consequences both from the standpoint of controlling the blast and the life of the porous elements.
To get around these difficulties, it has been suggested that monolithic elements with an oriented permeability be manufactured, by passing through an intermediate step where a naturally nonporous refractory piece is formed but which temporarily has on the inside a close network of links oriented in the direction of the blast, and which is later destroyed, leaving in its place an oriented network of fine multiple channels (French Pat. No. 1.271.201). These pieces appear to be satisfactory so far as their use is concerned, but it is in their manufacture that difficulties appear, because they require a complex and delicate equipment whose characteristics suitable for industrial production apparently have not yet been defined precisely.
Finally, this permeability must be sufficiently great so that the gas flows are not limited too much in view of the pressures that can be provided by the usual pneumatic installations in this field which, to give an idea of the figures involved, are usually around some 10 bars of relative pressure, approximately. As we can readily imagine, the greater the permeability of the porous piece, the less it is "selective" and the more it is subject to rapid wear by erosion in contact with the liquid metal. Therefore, one is faced with contradictory requirements, for which the solutions proposed up to this time have been compromises that are not always satisfactory, so far as is known to the inventors.
As an indication, it seems to have been in the field of ladle metallurgical treatment (addition, bringing up to grade, etc.) that the porous refractory pieces mentioned previously were first applied. In this case, since the required flow rates for the rabbling gas are relatively low (of the order of 5 liters per piece, approximately), the permeable pieces used generally have fairly good characteristics of selectivity and strength, so that their rate of wear is just about the same as that of the surrounding refractory lining.
On the other hand, in the case of very large capacity containers such as melting furnaces or converters, since the flow rates of the gas blown in are higher (about 10 times more), the pieces employed have to be highly permeable. This is correlated with the fact that their "selectivity" is impaired, making it impossible, as a general rule, to have a discontinuous blowing operation. Furthermore, mechanical wear is accelerated and becomes much more rapid than that of the refractory lining, which is even more unacceptable when we consider that the bottoms are designed to last for a much greater number of heats than the ladles, and it is presently out of the question to consider replacing a worn out piece during a campaign.